An etalon optical filter is essentially an optical cavity made of two partially transmitting optical surfaces that are approximately parallel to each other. The filter works by a principle of multiple beam interference. Various bounces of light off the optical surfaces add together to give a final transmitted or reflected beam. The net transmission or reflection is a function of wavelength and depends on the material properties used in coatings of the optical surfaces and the separation distance of the two optical surfaces.
If the two surfaces of the etalon have a high reflectance, then the bandpass of the filter will be narrow and referred to as having a high finesse. If the reflectance is low, then the etalon will allow a wider bandpass, or have a low finesse.
Metal coatings have been used in etalons and can be made to provide a fairly uniform response as a function of wavelength. However, a metal coating may not have a high reflectance unless it is very thick. Thick metal coatings also tend to absorb light, leading to a low net transmission.
In other prior devices, a separately generated dielectric coated etalon may be bonded onto a MOEMS (micro optical electrical mechanical system) device so that etch and deposition steps may be done on the MOEMS device separately. This may add steps to the fabrication of the device and result in difficult alignment and bonding of the etalon to separate filters on the original device. Increased cost and reduced yield may result.